1. Field of the Invention
The present invention relates to a hydraulic control apparatus of an automatic transmission mounted in a vehicle or the like, and particularly, relates to a hydraulic control apparatus of an automatic transmission which, while enabling an achievement of a speed stage at a time of a failure causing a de-energization (a failure time), has resolved a problem which is likely to occur at a normal time due to the failure.
2. Description of the Related Art
In general, with a multi-stage automatic transmission mounted in a vehicle or the like, each speed stage is formed by controlling a rotation condition of each rotational element of a speed change gear mechanism using an engagement condition of a plurality of frictional engagement elements, and the engagement condition of the plurality of frictional engagement elements is controlled by an engagement pressure being electrically adjusted using a solenoid valve and supplied to a hydraulic servo of each frictional engagement element.
Meanwhile, with the heretofore described kind of automatic transmission, when a failure of no electricity being supplied to the solenoid valve, that is, a so-called solenoid all-off (hereafter referred to a “failure” as appropriate) condition, occurs, the heretofore described electrical speed change control using the solenoid valve is disenabled. It is conceivable that this kind of failure condition is caused by, for example, a shutdown of a controller (an ECU), or a disconnection or short circuit of a battery wiring.
For this reason, an automatic transmission is proposed which, when this kind of failure condition occurs while travelling (in a forward range) (when a failure occurs), continuously achieves a securement of a travelling performance by achieving two kinds of speed stage in accordance with a speed stage before the failure condition has occurred without using the electrical pressure adjustment control of the solenoid valve (refer to JP-A-2005-265101). That is, the automatic transmission is configured in such a way as to achieve a third forward speed stage when a failure occurs in a first forward speed stage to a fourth forward speed stage, and a fifth forward speed stage when a failure occurs in the fifth forward speed stage to a sixth forward speed stage, that is, it is configured in such a way as to secure a travelling performance in a low speed stage when a failure occurs while travelling at low speed, and secure a travelling performance in a high speed stage when a failure occurs while travelling at high speed. The automatic transmission is configured in such a way as to, for example, attain the third forward speed stage when a manual shift valve is operated to switch from a drive position to a neutral position and to the drive position again after a failure occurs, and also has a so-called limp home function of this configuration enabling the vehicle to start moving again.